Geology of the Ice Age National Scenic Trail

Wisconsin Valley Lobe Ice Age Trail Segments

T he Wisconsin Valley Lobe advanced into northern Wisconsin out of the Lake Superior basin between 30,000 and 25,000 cal. years ago (fig. 1). It deposited reddish-brown, sandy, gravelly till (SB 5) and supraglacial sediment (SB 11). Look for the boulders strewn on the ground surface as well as the moraines (SB 6) that this advance left behind: the Harrison Moraine on the east and the Wood Lake Moraine on the west side of the lobe (figs. 216, 217). Outside the Harrison and Wood Lake moraines are glacial deposits that predate the late Wisconsin Glaciation. Radiocarbon dating shows that the Shelke bog, which is organic sediment under sand but on top of older till in eastern Lincoln County, formed between 36,000 and 40,800 radiocarbon years BP, so the till beneath it is even older than that (Mickelson 1986). The landscape covered by this earlier glaciation has been eroded, and most glacial landforms are gone. However, there is what appears to be older hummocky topography along the west side of the Wisconsin Valley Lobe. In places it is hard to distinguish exactly where the late Wisconsin advance reached its maximum extent in that landscape (Ham and Attig 1997; Attig 1993). There are no radiocarbon dates that can give us a precise time frame for the late Wisconsin advance of the Wisconsin Valley Lobe, but we can use the crosscutting relationships of moraines to figure out the relative ages of the advances (fig. 217). Drumlins (SB 14) in northern Lincoln County show there was ice flow from the northwest, but a narrow zone in back of the Wood Lake Moraine indicates a flow of ice to that moraine from the northeast (fig. 217). Therefore, the earliest advance was from the northeast to the Wood Lake Moraine, and a later flow from the northwest removed the evidence except in the zone close to the Wood Lake Moraine. It is possible that debris-covered ice existed in the Wood 282 Wisconsin Valley Lobe Ice Age Trail Segments Figure 216. Shaded relief of the Wisconsin Valley Lobe IAT segments (red): (90) Harrison Hills, (91) Alta Junction, (92) Underdown, (93) Grandfather Falls, (94) Turtle Rock, (95) Averill–Kelly Creek Wilderness, (96) Newwood, (97) Camp 27, (98) Timberland Wilderness, (99) Wood Lake. Yellow lines and numbers indicate major highways. Cities and villages shown (yellow): (M) Merrill, (T) Tomahawk. OW: outwash (SB 8). Blue arrows show ice-flow direction. Dashed blue line indicates outer extent of late Wisconsin glacial advance. (Base map constructed from USGS National Elevation Dataset and modified by WGNHS.) Lake Moraine when ice readvanced from the northwest to the Harrison Moraine, producing the northwest-to-southeast-oriented drumlins. We can also tell that the Harrison Moraine formed after the ice retreated from the Parrish Moraine, but before the Langlade Lobe advanced to the Summit Lake Moraine (fig. 201) because of the crosscutting relationships. It has been suggested that the advance to the Harrison Moraine was a glacial surge, a rapid advance of part of a glacier that is often caused by changes in water pressure at the glacier bed (Ham and Attig 1997). Modern surging glaciers often accumulate a thick layer of sediment on their ice surface, particularly near their edges, resulting in high hummocks (SB 11) and deep kettles (SB9) when the ice melts away. This might account for the higherrelief hummocky topography along the Harrison Hills and Underdown segments of the IAT, which are on the east side of the lobe. The hummocky topography in these two IAT segments is a world-class example of this type of landscape. There are ice-walled-lake plains (SB 15) among the irregular hummocks , kettles (SB 9), and disintegration ridges (SB 11). If you were to dig into these lake plains, you would find fine-grained lake sediment (sand, silt, and clay). You can pick out the ice-walled-lake plains because they often occupy high points in the landscape and they are commonly farmed. The Grandfather Falls, Turtle Rock, and Averill–Kelly Creek Wilderness segments of the IAT are in a more central part of the lobe, behind the end moraines. In these segments 284 Wisconsin Valley Lobe Ice Age Trail Segments Figure 217. Crosscutting relationships of moraines in north central Wisconsin. (Modified from Ham and Attig 1997.) you’ll hike through lower-relief hummocky topography located near the southern edge of a group of northwest...